As I interpret the "JMS Memo" detailing his original plan for B5, it seems as though the Shadows would have won the war against the Vorlons

Nothing in the outline says the Shadows win the war against the Vorlons; it just says that hundreds of innocent Vorlons are killed. (A philosophical aside; how many Vorlons are really "innocent"?) After Babylon 5's destruction, the outline states the Vorlons are one of the groups on the chase after Sinclair.

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Originally Posted by Republibot 3.0

So were the Drakh just placeholders for the Shadows once he wrote them out, or was there something specificly different that he needed to use them for?

I think the idea for the Drakh was there early on; "The Long Dark" as early as season two shows that the Shadows have servant races.